1. Field of the Invention
The present invention relates to the removal of water and CO.sub.2 from atmospheric air and is particularly concerned with the removal of these components from air to be employed as feed gas to a cryogenic air separation plant.
2. Prior Art
In the conventional process for cryogenic separation of air to recover nitrogen and oxygen, the feed air is compressed, then cooled to low temperature before introduction to a two stage distillation column. Unless water and CO.sub.2 are removed from the air after compression, these components will block the heat exchangers employed for cooling the gas prior to its distillation. The two principal methods for such removal are thermal swing adsorption (TSA) on molecular sieve, or controlled freeze out in reversing heat exchangers. Each of these systems presents its own advantages and disadvantages over the other from the standpoint of operation efficiency and economics. In most instances, reversing exchangers have been usually preferred unless it was required to obtain high nitrogen recovery which is better achieved by the thermal swing adsorption system. Such pretreatment of air prior to cryogenic separation is disclosed, for example, in U.S. Pat. Nos. 2,968,160 and 4,030,896.
In a typical prior art system employing molecular sieve or other solid sorbent for removal of water and CO.sub.2 from feed air, atmospheric air is compressed to about 100 psia (.about.7 bars) followed by water cooling, and removal of the thus condensed water. Then the air, which is then at about 100.degree. F. (.about.38.degree. C.), is further cooled to 40.degree. F. (.about.4-5.degree. C.) using refrigerated ethylene glycol. The bulk of the water is removed in this step by condensation and separation of the condensate. The gas is then passed to a molecular sieve bed system where the remaining water and CO.sub.2 are removed by adsorption. The sorbent beds are operated by the thermal swing mode with equal periods, such as four hours, being devoted to adsorption and to regeneration. In other systems, the time period for heating is shorter than that for subsequent cooling (U.S. Pat. No. 4,030,896). By using two beds, one is operated on adsorption while the other is being regenerated and their roles are periodically reversed in the operating cycle. During the regeneration, part of the product gas (nitrogen) is slightly compressed with a blower and then initially heated to say about 500.degree. F. (260.degree. C.). The hot gas is passed through the bed then being regenerated, say for two hours, following which the regeneration gas is cooled to say 40.degree. F. (4.degree.-5.degree. C.) for the final two hours for cooling the bed to that temperature. Regeneration is carried out in a direction counter to that of the adsorption step.
Pre-treatment of air for removal of water and CO.sub.2 is also known in connection with adsorptive systems for separation of oxygen and nitrogen therein. The preliminary removal of the water and CO.sub.2 may be carried out in a single column containing a single bed of adsorbent, or such column containing separate layers of adsorbents selective respectively for water and CO.sub.2. It has also been suggested to employ separated beds for removal of water and CO.sub.2 respectively from the feed air.
In systems wherein the pretreatment section for water and CO.sub.2 removal is employed in association with an adsorptive air separation section operating in the pressure swing adsorption (PSA) mode for regeneration of the main adsorber columns therein, the pretreatment section may also be operated in the pressure swing mode as in U.S. Pat. Nos. 3,796,022; 4,013,429; and 4,042,349. It has also been proposed to operate the pretreater section by periodic thermal regeneration, while the N.sub.2 --O.sub.2 separation section is operated in the pressure swing model; U.S. Pat. Nos. 3,140,931; and 3,533,221.
The removal of water and CO.sub.2 from atmospheric air is not limited to situations in which the purified air is to be employed as feed to a cryogenic air separation plant, or as feed to a selective adsorption section for N.sub.2 --O.sub.2 separation. For example, pretreatment of hydrocarbon streams for removal of water and CO.sub.2 prior to liquefaction of such streams is disclosed in U.S. Pat. No. 3,841,058. The water (plus methanol) and the CO.sub.2 respectively are removed from the feed in separate consecutive adsorbent beds. The water-laden bed is thermally regenerated and the CO.sub.2 -laden bed is regenerated by pressure reduction, with or without some warming.
U.S. Pat. No. 3,594,983 is particularly concerned with the removal of CO.sub.2 contained in relatively large amounts in a natural gas stream which also contains relatively small amounts of water and H.sub.2 S. Three adsorbent columns are employed which operate in parallel. Regeneration is effected by pressure swing carried out in alternating short interval sequence in two of the columns for removal of adsorbed CO.sub.2. After a number of repeated thermal regeneration cycles previously undergone by the third column, that column is subjected to thermal desorption of the more strongly sorbed water therefrom.
The purification of air withdrawn from a closed compartment and the return of the purified air to that compartment is exemplified in U.S. Pat. Nos. 3,221,477; 3,242,651; and 3,738,084. Here again, separate beds are used for removal of the moisture and CO.sub.2 respectively from the feed air. The purified gas may be heated and passed through the moisture-laden bed to regenerate that bed and restore moisture to the purified gas returned to the compartment. The CO.sub.2 -laden bed may be regenerated during a separate independent time interval. In U.S. Pat. No. 3,738,084, regeneration is carried out employing both pressure reduction and heating of selected beds.